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What happens when you stop eating is genuinely interesting, and most of it is well described. This section covers the physiology one process at a time: the fuel your body switches to, the signals that drive the switch, and how each claim is measured.
Every page here is drawn from peer-reviewed research, traced to a dated evidence packet, and kept separate from Burn's own stage labels.
Claims come from peer-reviewed research, recorded in an evidence packet with its sources, search date, and review date.
Burn's nine stages are product labels for elapsed time. Where a page mentions them, it says so plainly.
Each page carries a review date, and corrections are published rather than quietly edited in.
Start here
Read these three in order and the rest of this section will make sense.
FOUNDATION
The whole mechanism in one place: insulin falling, glycogen draining, fat taking over, and what that adds up to.
Read the explainer →MEASURABLE
What ketones are, what a blood test actually tells you, and why the onset hour varies from person to person.
Read about ketosis →MOST DEBATED
Cellular recycling, where the evidence comes from, and why measuring it takes a laboratory.
Read about autophagy →The fuel switch
Six pages, in the sequence your body works through them. Each one is a single process rather than a grand claim.
01
Two states, and what separates them.
02
The easy fuel, and how long it lasts.
03
Making glucose when none is coming in.
04
Releasing stored fat into the bloodstream.
05
Burning it, and how the rate changes.
06
Moving between fuels, and what it costs.
Everything in this section
What your body runs on, and when it changes.
The part of a fast you can actually measure.
Cellular recycling, and the limits of what is known.
The machinery that responds to going without food.
The science pages cover one process each. If you want to know what is happening at a specific point in a fast, the stage pages walk through it in order.
Go to fasting stages →Our editorial standards set out where claims come from, who reviews them, and how corrections are published.
Read our editorial standards →Burn names all nine stages as you reach them and tells you how firm the evidence behind each one is.
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